SEXUAL CONFLICT AND THE ENERGETIC COSTS OF MATING AND MATE CHOICE
IN WATER STRIDERS

ABSTRACT

Analyses of intersexual conflicts-of-interest over courtship, mating or mate-guarding require an
understanding of the physiological costs of sexual interaction. Repeated respirometric measures
of energetic expenditure were taken on female Aquariusremigis while unladen, and while
carrying a mating male, a small metal weight or a euthanized male. Unladen "cruising"
locomotion consumed an average of 334.6 microwatts (uW) of energy (82 J kg-1 m-1); this
estimate of the cost of pedestrian locomotion is moderately low compared to other insects of similar mass
and represents the first energetic measure of skating on a water surface. Cruising females
carrying males or metal weights consumed 24% and 28% more energy than unladen females,
respectively. Females engaged in "escape" locomotion consumed 43% more energy while
carrying a male than while unladen. Further, our study shows that pre-mating struggles, and
therefore selective mating decisions, are energetically costly. Struggling females consumed an
average of 936.6 uW, a 126% increase compared to cruising, non-struggling females, and 64%
more than mating females engaged in escape locomotion. We develop a quantitative model
showing that at a certain harassment rate threshold, accepting superfluous matings becomes the
"best of a bad job" for females.